When casting concrete beams formed internally with at least one longitudinal channel in upright moulds it is known to use longitudinal, inflatable hoses of rubber, soft plastics or some similar elastic and flexible material, which hoses are placed in the mould and clamped in position during the casting process to prevent them from being lifted from their predetermined position inside the mould by the concrete introduced into the mould. After the casting, the hoses are deflated and then pulled out of the cast object. When concrete piles or similar concrete structural members, such as concrete flooring slabs, of the type having internal longitudinal channels, are manufactured in horizontal moulds, tubes are used as the hollow means, which tubes are inserted into and retained in the mould whereafter the concrete is poured into the mould so as to surround the tubes. As the tubes are left inside the cast objects, the costs thereof are considerably increased without attributing to any significant degree to the durability and strength of the product. To use an inflatable hose as the hollow means has proved impossible in these cases, because the concrete, when poured about the hose, lifts the hose from its intended position.